1. Field of the Invention
This invention relates to apparatus for handling semiconductor wafers to facilitate making electrical tests on semiconductor devices formed in a matrix arrangement consisting of a large number of identical devices, each of which has up to 20 contact areas of microscopic size, on the surface of a semiconductor wafer.
2. The Prior Art
Semiconductor devices are formed on the surface of a semiconductor wafer approximately 7 or 8 centimeters in diameter divided into a matrix of individual units having length and width dimensions of approximately 1 to 4 millimeters. All of the devices formed on a single semiconductor wafer are identical with each other and each of them has a number of contact pads to which lead wires will eventually be connected in further manufacturing steps after the wafer has been divided into the individual semiconductor chips, each of which corresponds to one cell of the matrix. Before the wires are connected, it is desirable to test each chip to determine that its electrical characteristics are correct according to its design, and such testing is preferably carried out before the relatively large semiconductor wafer is broken up into the individual chips.
Testing apparatus may be in the form of a machine that has means for holding the wafer and for moving it precise distances corresponding to the dimensions of each of the chips thereon. At each position up to 20 contacts must be brought into engagement with the contact pad areas of each chip. These pad areas have dimensions on the order of 100 microns .times. 100 microns, and they may be spaced apart by a generally similar order of magnitude. Thus the precision of placement of the contacts on the contact pads is of a high order of exactness.
Not only is it necessary that the contacts be accurately placed on the pads of a given chip area, but the direction and distance of movement to bring those contacts into proper relation with additional chips in the matrix requires very accurate angular alignment between the matrix grid directions and the directions of movement of the testing apparatus. This is necessary to prevent the testing contacts, which might be making acceptable connections with the first chip at one corner of the matrix, from drifting away from the proper contact pad areas as the apparatus moved along the matrix rows and columns.